1. (a) Field of the Invention
The present invention relates to a magneto optical recording medium having a perpendicular magnetic anisotropic thin film made of a hexagonal magnetoplumbite-type metal oxide magnetic substance.
2. (b) Description of the Prior Art
In recent years, magneto optical recording mediums designed to write a domain in a perpendicularly magnetized magnetic thin film utilizing the thermal effect of light and record information, and read said recorded information out utilizing the magneto-optic effect have been attracting public attention.
As the magnetic substances used in magneto optical recording mediums, there have usually been known those consisting of amorphous alloys made of rare earth metals and transition metals. The magneto-optical recording mediums using such amorphous alloy magnetic substances are generally prepared in the manner of adhering said magnetic substance, for instance Tb-Fe alloy, on a substrate such as a glass plate by vacuum vapordeposition, sputtering or the like in the degree of about 0.1-1 .mu.m in thickness to form a magnetic substance. Record of information is put on this magneto-optical recording medium in the manner of making use of the rapid variation characteristic of a coercive force corresponding to the temperature variation in the neighborhood of the Curie temperature or compensation temperature of the magnetic substance, concretely in the manner of radiating the laser beam modulated with a divalent signal on the magnetic film and heating, thereby inverting the direction of magnetization. Regeneration is carried out by reading out making use of the difference in magneto-optical effects of the thus inversion-recorded magnetic film. The magneto-optical recording medium using the aforesaid amorphous alloy magnetic substance is advantageous in that it can be prepared easily by vacuum vapordeposition, sputtering or the like, and recording can be carried out at a high speed with a semi-conductor laser beam because the curie temperature is relatively low, namely 70.degree.-200.degree. C. and the recording sensitivity is high, but is very defective in that the amorphous alloy magnetic substance, in particular the rare earth metal component, is subject to oxidizing corrosion, and consequently the magnetooptic characteristic of the magnetic film deteriorates with a lapse of time. In order to prevent this, it is known to provide a protective layer consisting of an oxide such as SiO, SiO.sub.2 or the like on an amorphous magnetic film. However, this is defective in that at the time of preparing the magnetic film or the protective layer by means of vacuum vapordeposition, sputtering or the like, the magnetic film is oxidized and corroded by O.sub.2 remaining in the vacuum, O.sub.2, H.sub.2 O or the like adsorbed onto the surface of the substrate, and O.sub.2, H.sub.2 O or the like contained in the target consisting of the alloy magnetic substance with a lapse of time, and that this oxidizing corrosion is further enhanced by the light and heat at the time of recording. Further, the amorphous magnetic substance is disadvantageous in that it is liable to be crystallized by heat and therefore its magnetic characteristic is liable to be deteriorated.
In the usual magneto-optical recording mediums, still further, the recorded information has been read out by means of the magneto-optical effect caused by reflection from the surface of the magnetic film, namely the Kerr effect, because this magnetic film is low in the permeability in the laser wavelength region. However, this involves the problem that the regeneration sensitivity is low because the Kerr's angle of rotation is generally small.
On the other hand, the inventors of this application has developed a magneto optical recording medium using a perpendicular magnetic film comprising a hexagonal magnetoplumbite-type metal oxide magnetic layer (Japanese Laid Open Patent Application No. 45644/1984, Japanese Laid Open Patent Application No. 168950/1984).
It has usually be known that hexagonal magnetoplumbite-type metal oxide magnetic substances represented by the general formula (A) are usable as magnetic bubble materials. EQU MeO.n(Fe.sub.2 O.sub.3) (A)
(wherein, Me stands for one kind or 2 kinds or more of metals selected from the group consisting of Ba, SR and Pb, and N:5.ltoreq.n.ltoreq.6)
The inventors of this application have perceived that the magnetic substance of this sort is a stable oxide and has no possibility of oxidizing deterioration, can display permeability in the laser waverlength region even when the film thickness is increased, and can utilize the Faraday effect efficiently. However, the above mentioned hexagonal magnetoplumbite-type metal oxide magnetic substance having the aforesaid general formula (A), even if planned to use said substance as it stands as the magnetic film for the magneto optical recording medium, involves the problems that the Curie temperature is too high, the value of coercive force is not suitable, the Faraday effect is small and the like, and is not reliable in the points of write-in ability, read-out ability, sensitivity and the like required for the memory. The inventors of this application have previously proposed to solve the above mentioned problems in the manner of replacing part of Fe atom having the said general formula (A) by divalent metals such as Co and Ni, trivalent metals such as In and Al, or further tetravalent metals such as Ti, Ge, W and Mo, thereby making it possible to use the hexagonal magnetoplumbite-type metal oxide magnetic substance as the magnetic film for the magneto optical recording medium.
In the case of using said hexagonal magnetoplumbite-type metal oxide magnetic film in the magneto-optical recording medium, it is necessary that said film should be the co-called perpendicular magnetic film wherein the C axis of the hexagonal crystal has been orientated on the surface of the substrate. The inventors of this application have previously found that in a magneto-optical recording medium comprising a substrate and a magnetic layer, there can be formed the magnetic layer having a perpendicular magnetic anisotropy by maintaining the misfit between a specific crystal surface of said substrate and the C surface of a hexagonal magnetoplumbite-type metal oxide magnetic substance within a predetermined range (Japanese Laid Open Patent Application No. 175229/1985), and have also proposed that the substrate of this application may be provided with an undercoat layer of ZnO, AlN or the like. In the thus obtained magneto-optical recording medium, however, the magnetooptical effect, increase in pit number, reproducibility and high density record regeneratability are influenced widely by the fine difference in the C axis orientation property of the hexagonal magnetoplumbite-type metal oxide magnetic layer, and consequently the crystalline orientation property of the substrate or undercoat layer on which this oxide magnetic layer is formed has an exceedingly great influence. It is difficult for this invention to maintain the crystalline orientation property of the substrate or undercoat layer always within a predetermined range. Accordingly, the above mentioned effects of the magneto-optical recording medium per se can not be achieved to the full, because the perpendicular magnetic anisotropy becomes unbalanced.